The present invention relates to magnetic acicular composite particles for magnetic recording medium, and a magnetic recording medium using the black magnetic acicular composite particles. More particularly, the present invention relates to magnetic acicular composite particles exhibiting not only an excellent dispersibility in vehicle due to a less amount of carbon black desorbed or fallen-off from the surfaces thereof, but also a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid-adsorption; and a magnetic recording medium exhibiting not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability.
With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly desired to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Especially, video tapes have recently been desired more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
In order to enhance output characteristics of magnetic recording media, especially an S/N ratio thereof with respect to signals having a short wavelength, there have been demanded fineness of magnetic particles, reduction in thickness of a magnetic recording layer, high dispersibility of magnetic particles and surface smoothness of a magnetic coating film.
On the other hand, at the present time, the end position of a magnetic recording medium such as magnetic tapes has been detected by sensing a high light transmittance portion of the magnetic recording medium by means of a video deck. In the case where the particle size of magnetic particles dispersed in the magnetic recording layer become finer and the thickness of the magnetic recording medium is reduced in order to meet the requirement for high performance of the magnetic recording medium as described hereinbefore, the magnetic recording medium shows a high light transmittance as a whole, so that it has been difficult to detect the end position thereof by means of the video deck. In order to solve this problem, carbon black fine particles have been added to the magnetic recording layer in an amount of usually about 6 to 12 parts by weight based on 100 parts by weight of the magnetic particles, thereby reducing the light transmittance of the magnetic recording medium. For this reason, in current videotapes, it is essential to add carbon black fine particles, etc., to the magnetic recording layer thereof.
However, when a large amount of such non-magnetic carbon black fine particles are added to the magnetic recording layer, the magnetic recording medium suffers from deterioration in signal recording property, thereby hindering high-density recording thereon, and the reduction in thickness of the magnetic recording layer becomes incapable. Further, due to the fact that the carbon black fine particles have an average particle size as fine as 0.002 to 0.05 μm and a large BET specific surface area value, and are deteriorated in solvent-wettability, it has been difficult to disperse these carbon black fine particles in vehicle, thereby failing to obtain a magnetic recording medium having a smooth surface.
Further, in the case where the magnetic recording medium has a high surface electrical resistivity, the electrostatic charge amount thereof is increased, so that cut chips or dusts tend to adhere onto the surface of the magnetic recording medium upon the production or use thereof, thereby causing such a problem that the dropout frequently occurs. Therefore, in order to reduce not only the light transmittance of the magnetic recording medium but also the surface electrical resistivity thereof, especially below about 108 Ω/cm2, the carbon black fine particles have been conventionally added to the magnetic recording layer of the magnetic recording medium.
However, as described above, in the case where the amount of such carbon black fine particles or the like which do not contribute to magnetic properties of the magnetic recording layer, is increased, there are caused such problems that the magnetic recording medium suffers from deterioration in signal recording property, the reduction in thickness of the magnetic recording layer becomes incapable, and further the surface smoothness of the magnetic recording layer is deteriorated.
Also, since the carbon black fine particles are bulky particles having a bulk density as low as about 0.1 g/cm3, the handling property and workability thereof are deteriorated. In addition, it has been pointed out that the use of such carbon black fine particles causes problems concerning safety and hygiene such as carcinogenesis.
Magnetic recording media have been continuously required to enhance performances thereof. In particular, it has been strongly required to improve physical properties of these magnetic recording media such as running property in addition to the above-described high-density recording property.
The running property of magnetic recording media (tapes) can be ensured by adding a fatty acid such as myristic acid or stearic acid (hereinafter referred to merely as “myristic acid”) in an amount of usually about 0.5 to 5% by weight based on the weight of magnetic particles, into a magnetic recording layer generally formed as an upper layer of the respective magnetic recording medium, and then allowing the myristic acid to be gradually oozed out on the surface of the magnetic recording layer so as to render the surface slidable.
When the amount of the myristic acid oozed out on the surface of the magnetic recording layer is too small, it is not possible to ensure a good running property of the magnetic recording media. On the contrary, when a large amount of the myristic acid is added to the magnetic recording layer so as to allow a large amount of myristic acid to be subsequently oozed out on the surface thereof, the myristic acid is preferentially adsorbed onto the surface of each magnetic particle dispersed in the magnetic recording layer, thereby inhibiting the magnetic particles from being contacted with or adsorbed into resins. As a result, it is difficult to disperse the magnetic particles in vehicle. Also, the increase in amount of the myristic acid as a non-magnetic component causes deterioration in magnetic properties of the magnetic recording media. Further, since the myristic acid acts as a plasticizer, there arise problems such as deterioration in mechanical strength of the magnetic recording media.
Recently, with further reduction in thickness of the magnetic recording layer, the absolute amount of myristic acid added to the magnetic recording layer is decreased. In addition, since the particle size of the magnetic particles have become much finer in order to meet the requirement of high-density recording, the BET specific surface area thereof is increased, so that a large amount of myristic acid is absorbed onto the surfaces of the magnetic particles. Under these conditions, it is more and more difficult to properly adjust the amount of the myristic acid oozed out on the surface of the magnetic recording layer by the amount of the myristic acid added into the magnetic recording layer, thereby ensuring a good running property of the magnetic recording layer.
Accordingly, it has been strongly required to properly adjust the amount of myristic acid oozed out on the surface of the magnetic recording layer and ensure a good running property of the magnetic recording media.
In order to improve electrical resistance and running durability of the magnetic recording media, there have been proposed such magnetic particles on the surfaces of which carbon and/or graphite are deposited or adhered in an amount of 0.2 to 10.0% by weight based on the weight of the magnetic particles (corresponding to 0.2 to 11.11 parts by weight based on 100 parts by weight of the magnetic particles) by various methods such as gas-phase growth, e.g., chemical vapor deposition (CVD) or physical vapor deposition (PVD), reduction of organic compounds, and thermal decomposition or incomplete combustion of hydrocarbons (Japanese Patent Application Laid-Open (KOKAI) No. 10-269558(1998)).
Also, hitherto, with the reduction in thicknesses of magnetic recording layer and non-magnetic base film of magnetic recording media, it has been variously attempted to impart good surface smoothness and large stiffness thereto by improving a substrate on which the magnetic recording layer is formed. For instance, there has been proposed a non-magnetic substrate composed of a non-magnetic base film and at least one undercoat layer formed on the non-magnetic base film. The undercoat layer is composed of a binder and non-magnetic particles dispersed in the vehicle, which contain iron as a main component, e.g., acicular hematite particles or acicular iron oxide hydroxide particles (hereinafter referred to merely as “non-magnetic undercoat layer”). Such a non-magnetic substrate is already put into practice (refer to Japanese Patent Publication (KOKOKU) No. 6-93297(1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993), 5-182177(1993), 5-347017(1993) and 6-60362(1994), or the like).
As non-magnetic particles for the non-magnetic undercoat layer, there have been known non-magnetic particles which are treated with a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon or an oxide of silicon in order to improve a dispersibility of these particles in vehicle, etc., for further smoothening the surface of a substrate and increasing the stiffness thereof (Japanese Patent Nos. 2,571,350 and 2,582,051, and Japanese Patent Application Laid-Open (KOKAI) Nos. 6-60362(1994), 9-22524(1997) and 9-27117(1997)).
Also, in order to reduce a light transmittance of magnetic recording media by reducing the amount of carbon black added to a magnetic recording layer thereof, it has been known to use as non-magnetic particles for non-magnetic undercoat layer, blackish brown acicular hematite particles or blackish brown acicular iron oxide hydroxide particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 7-66020(1995), 8-259237(1996) and 9-167333(1997) or the like). Further, it has been known to use non-magnetic acicular particles which are composed of acicular hematite particles or acicular iron oxide hydroxide particles as core particles and carbon black fine particles adhered onto the surfaces of the core particles in an amount of 1 to 20 parts by weight based on 100 parts by weight of the core particles (European Patent No. 0,824,690 A).
Also, it has been known that mixed particles composed of non-magnetic iron oxide particles and carbon black particles are used as non-magnetic particles for non-magnetic undercoat layer in order to reduce the surface electrical resistivity value of magnetic recording media (Japanese Patent Application Laid-Open (KOKAI) Nos. 1-213822(1989), 1-300419(1989), 6-236542(1994) and 9-297911(1997) or the like).
At present, it has been strongly demanded to provide magnetic recording media which exhibit not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability. However, magnetic recording media satisfying all of these properties have not been obtained until now.
As a result of the present inventors' earnest studies in view of the above problems, it has been found that by using as magnetic particles for magnetic recording medium, black magnetic acicular composite particles comprising:                magnetic acicular cobalt-coated iron oxide particles or magnetic acicular metal particles containing iron as a main component;        a coating formed on surface of the magnetic acicular particle, comprising at least one organosilicon compound selected from the group consisting of:        (1) organosilane compounds obtainable from alkoxysilane compounds, and        (2) polysiloxane or modified polysiloxane; and        a carbon black coat formed on the coating layer comprising the organosilicon compound, in an amount of from more than 10 to 40 parts by weight based on 100 parts by weight of said magnetic acicular particles,        the obtained magnetic recording medium can exhibit not only a smooth surface, a smaller light transmittance and a lower surface electrical resistivity value, but also a small friction coefficient and an excellent running durability. The present invention has been attained based on this finding.        